In many dynamic systems, there is sometimes created a destructive deceleration which develops within a relatively short distance. While in certain circumstances, a compartment subject to distortion may provide an adequate survival space, nevertheless, the human body cannot withstand deceleration forces, above a certain limit.
The efficiency of an energy absorbing device is given by the ratio: EQU SEA (specific energy absorbed)=energy absorbed/device weight.
A common unit of specific energy is J/g. (Joules/gram). The desirable features of an energy absorbing device are as follows:
it should provide a predictable FORCE vs. DEFORMATION trace; PA1 the rapid loading rate expected in crashes should not change the FORCE vs. DEFORMATION behavior; PA1 it should operate under both tension and compression; PA1 the device should be as light and as small as possible; PA1 the specific energy absorption (SEA) should be high; PA1 it should be economical to manufacture; PA1 it should be reliable and maintenance-free for a long period of time; PA1 it should not be adversely affected by dirt, corrosion or other environmental factors; PA1 the person involved should be decelerated in the most efficient manner possible, while maintaining the loading environment within the limits of human and/or payload tolerance; PA1 its stroke should be relatively long as compared to its total length (i.e. a high stroke efficiency).
There are many energy absorbing devices for impact applications, the most common ones utilize deformation of metals and friction to dissipate energy. Some examples are as follows:
______________________________________ Device SEA(J/g.) ______________________________________ axial compression of a steel tube 25 axial compression of an Al tube 16 steel strap/wire over die or roller 4 steel inversion tube 4 basic elongation of steeI 14 crushing a rigid foam 20 steel rod pulled through a tube 2 tube flaring 3 controlled crushing of a metal tube 45 controlled crushing of structural honeycomb 40 controlled axial crushing of a composite tube with Kevlar(R) reinforcing fiber 20 with glass reinforcing fiber 60 with carhon reinforcing fiber .ltoreq.100 ______________________________________
It should be noted that in the last three cases (controlled axial crushing of a composite tube) the reported SEA values refer to the absorbing material and not to the entire device. Moreover, carbon fibers are relatively expensive. In addition to the above examples, there exist energy absorbing devices based on an elastic element such as springs, compressed gas and fluids. However, their stroke is short, the force increases and they become almost rigid. Their strong recovery and bounce back are undesirable. In some cases (pneumatic/hydraulic shock absorbers) a valve forces fluid out at a predesigned pressure; such devices have SEA in the range 5 to 25 J/g.
In U.S. Pat. No. 4,523,730 (Martin), there is disclosed an energy absorbing seat arrangement, particularly for a helicopter, in which a seat pan is carried by a frame slidably mounted on parallel upwardly extending pillars secured to the helicopter. Normally, downward sliding of the frame on the pillars is prevented either by metal rods extending through drawing dies, or by a deformable metal tube and a mandrel extending through the tube. In the event of a crash, the rods are drawn through the dies or the mandrel through the deformable tube. However, this metal/metal arrangement, like other similar arrangements, suffers from a relatively low SEA, the fact that it is very restricted as to its potential for cross-sectional area reduction (maximum elongation of a ductile stainless steel is only 45-50%), dependence on metal/metal friction to maintain a uniform load is unpredictable, while oxidation may alter the properties of the metal/metal interface. As will be seen, the present invention does not utilize a metal/metal arrangement and thus avoids such problems.
U.S. Pat. No. 3,865,418 describes an energy absorbing device for a vehicle including a cylinder having a stepped inner diameter and in which an annular plastic slug is extruded between the cylinder and a stepped ram. The stroke efficiency is less than one half of the length of the device.
U.S. Pat. No. 2,997,325 to Peterson describes a kinetic energy absorber in which a piston forces an extrudable body through a nozzle. U.S. Pat. No. 3,380,557 describes a variable kinetic energy absorber in which a piston serially forces a plurality of extrudable bodies, each having an increased resistance to extrusion, through a nozzle. Both of these patents employ a cylinder formed of heavy metal in order to withstand the high pressure produced during extrusion. The proposed structure is relatively heavy and therefore the absorber has a relatively low SEA. The piston stroke is limited to the length of the cylinder and thus the stroke efficiency of the absorber is limited to less than one half of the overall absorber length. Furthermore, during stroking, the friction force between the extrudable plastic material and the cylinder wall decreases producing a consequent reduction in the stopping force of the absorber, as the stroke progresses.
U.S. Pat. No. 3,532,380 describes an energy absorbing device for a restraint belt. GB Patent Application 1,506,157 describes an energy absorbing device employing a piston and extrudable material which is similar to that described in U.S. Pat. No. 3,865,418 but employs a smooth cylinder. UK. Published Patent Application 2,048,430 describes a device absorbing energy by extrusion.